Jami` at-Tirmidhi Book 17, Hadith 23
That the Messenger of Allah (ﷺ) said: "If one of your slave girl commits illegal sexual intercourse, then whip her three times according to the Book of Allah, and if she does it again then sell her, even if it is for a rope made of hair."
"We were with the Prophet (ﷺ) partners with Allah, and that you will not steal nor commit adultery.' He recited to them the Ayah. (And he said:)'Whoever among you dies, then this reward is with Allah, and whoever among you does some of this and then he is punished, it is atonement for him. And whoever does some of this and Allah covers it for him, then it is up to Allah; if He wills, He will punish them, and if He wills, He will forgive him.'"
"Ali gave a Khutbah, and said: 'O people, establish the penalties upon your slaves, those married from them and those unmarried. A slave girl of the Prophet (ﷺ) committed illegal sexual intercourse so he ordered me to whip her. I went to her and she was just experiencing her post-natal bleeding, so I feared that if I were to whip her I would kill her' - or he said: 'She would die' - 'so I went to the Messenger of Allah (ﷺ) and I told that to him. So he said: 'You did well.'"